The long term goal of this renewal grant application is to understand the molecular mechanism of host-virus interactions and gene expression of human parainfluenza virus, type 3 (HPIV3), an important human pathogen which targets lung epithelial cells of the respiratory tract of infants and children leading to acute bronchiolitis, pneumonia and croup. Effective vaccines or antivirals to combat HPIV3 infection are currently unavailable. Our primary goal is to gain insight into some specific host-virus interactive processes which include, (b) identification and characterization of the cell surface receptors of epithelial cells (a) the role of actin binding protein, beta catenin in the transcription ability of HPIV3 ribonucleoprotein (RNP) complex and (c) regulation of expression of major histocompatibility complex II induced by HPIV3 which may play an important role in cellular immunity leading to infection-related damage to lung epithelium. We also propose to continue our studies in understanding the structure and function of the RNA polymerase, L and P proteins, of HPIV3, especially with regard to the role of phosphorylation of P protein and identification and characterization of the functional domains of L and P proteins. Finally, we wish to exploit reverse genetics using the infectious cDNA of HPIV3 constructed in our laboratory to study phenotypic characteristics of some distinctive cis-regulatory mutants. The studies proposed in this application have the potential to gain deeper insight to various host-virus interactive pathways, which may eventually lead to rational designing of antivirals, vaccines, and expression vectors.